EP1598446A2 - Process and system for detecting alternating stray current corrosion in cathodically protected plants - Google Patents

Abstract

An arrangement for detecting AC corrosion at portions of cathodically protected equipment comprises a first electrode (1) and a further electrode (3). A transducer (6) for an electrical quantity is electrically coupled to the first electrode (1). Between the first electrode (1) and the second electrode (3) is an electrolyte (4), so that the electrodes (1, 3) are in electrolyte communication. Between the first electrode (1) and the second electrode (3) a voltage source (5) is arranged to initiate an electrochemical reaction at the first electrode (1). Furthermore, a reference electrode (2) is provided which is in electrolyte communication with the first electrode (1) via the electrolyte (4). The first electrode (1) is a section of a cathodically protected pipeline.

Description

The present invention relates to a method and an arrangement for Acquisition of AC corrosion on cathodically protected equipment, with a to be examined section of the plant contains a corrodible material which reacts when AC corrosion occurs to a corrosion product.

By cathodic corrosion protection, buried pipelines and other installations in corrosive environment, known to be protected against corrosion. However, induced AC voltage in the systems may cause it cathodic corrosion protection come to AC corrosion.

The control of buried facilities for corrosion is problematic. This is mainly due to the unfavorable accessibility of the plants to be protected. It is a method of determining the pipeline bottom potential at cathodic protected pipelines, where at different Measuring points along a pipeline a reference electrode above the pipeline is placed on the ground (DE 100 21 994 A1 - Method of determination of the tube / ground potential on cathodically protected pipelines). This Method is for the detection and quantitative detection of AC corrosion but not suitable.

For the control of the corrosion state of pipelines are further Test pieces, so-called coupons, buried in the area of the pipeline and electrically connected to this. The coupons are mostly coated Steel sheets with a target defect, in the coating. The latter is for the Corrosion state of a defect in the adjacent insulated pipe representative. Measurements of the AC current density at this target defect allow conclusions about the occurrence of AC corrosion. By the However, known method is only an instantaneous detection of AC density possible, but not a picture over a long term to gain variable corrosion state of the pipeline.

Furthermore, potential measurements on the coupons are in the form of so-called High-speed measurements known. The coupons are from the pipeline electrically decoupled, and the potential of the coupon is immediately after the Separation against a standard potential measured.

All known methods do not allow detection of the Corrosion state of real defects on pipelines.

Object of the present invention is to provide a detection and quantitative Acquisition of AC corrosion on cathodically protected installations improve.

Based on a method of the type mentioned, this object is achieved in that from a portion of the cathodically protected system, a first electrode is formed, which contains a quantity of the corrosion product and the first electrode via an electrolyte with at least one further electrode in electrolyte compound is brought
in that an electrical voltage is applied between the first and the at least one further electrode in order to initiate an electrochemical reaction,
wherein the corrosion product of the first electrode participates in the electrochemical reaction and is converted to a reaction product,
that the course of the reaction is controlled by adjusting at least one electrical quantity at the electrodes,
that at least one electrical measured quantity is detected from a group of measured variables consisting of current intensity, potential difference and charge, and
the amount of the corrosion product reacted in the reaction is determined from the at least one electrical measured variable.

The object is further according to the invention by an arrangement according to Claim 14 solved.

The selection of the electrical voltage source may vary according to the circumstances Site and the desired measurement times are taken. The measured quantities must draw conclusions on the type and / or quantity of the implemented Allow corrosion product. For this purpose, electric power or Voltage sources in the form of a galvanostat, a potentiostat, a battery or a DC power supply. With the help of a galvanostat For example, a corrosion product may be at a given regulated current to be implemented, taking care to maintain the amperage required voltage is measured. From the amperage is in conjunction with the measuring time to determine the flowed charge and knowing the nature of the Corrosion product by coulometric calculations, the amount of substance determined. With knowledge of the active electrode surface is also a surface-related Quantity of substance or area-related amount of charge determinable.

Alternatively, a predetermined voltage can be applied to the electrodes and the amperage is detected. Thus, for example, the reaction to the Implementation of a type of corrosion product to be limited Implementation voltage is known.

It is advantageous if, in addition to the detection, the at least one electrical quantity recorded a measuring time and the amount of Corrosion product is determined taking into account the measurement time.

The measuring time allows further evaluations of the electrical measured variable. For example, for evaluation a temporal measured variable course with a be compared under known conditions reference measurement. Furthermore, by temporal integration of the measured current strength, the calculation the charge flow possible.

When using a pipe section to be tested Temporarily connected test pieces (coupons) may be the measurement of the test piece contained corrosion product at the place where the coupon at the Plant is arranged (e.g., buried) plant. The coupon is usually for the Period of measurement electrically disconnected from the system. The coupon can Alternatively, be removed from the section of the system and in one Measuring electrolytes are measured in a test setup.

In an alternative procedure, as the first electrode, a section of a cathodically protected piping can be used. The electrochemical Reaction does not find an artificial defect, but a real one Defective place of the pipeline. The electrolyte used in this case is usually the the pipeline surrounding medium (e.g., soil, seawater).

Preferably, to control the electrochemical reaction three Electrodes in a three-electrode electrochemical structure with working, and reference electrode, wherein between the working electrode and the Reference electrode, an electrical potential difference is measured.

Such a measuring structure is known from physical chemistry or electrochemistry. The three electrodes are coupled such that the reference electrode is in electrolyte communication with the working electrode and an electrical potential difference is measured between these two electrodes. Since the connection between the working electrode and the reference electrode usually remains currentless, this measurement is not affected by current flow. A current flow takes place only between the working electrode and the counter electrode. As the reference electrode, commercially available reference electrodes such as Ag / AgCl or Cu / CuSO _{4 may be used} .

Preferably, in at least a partial reaction of the electrochemical reaction, oxidation of Fe ²⁺ to Fe ^{3+ is} carried out.

Since the majority of cathodically protected equipment contains iron, the Occurrence of AC corrosion often metallic iron to divalent iron ions oxidized. In the area of buried, cathodically protected Plants are usually alkaline conditions. Therefore, forms from the Iron ion poorly soluble iron hydroxide, which is located in front of the metal surface accumulates. In the presence of oxygen, the divalent iron hydroxide would become continue to react with trivalent iron hydroxide; under the influence of the cathodic Corrosion protection, however, this oxidation reaction is usually prevented. This partly because the environment of the defect is depleted of oxygen, since this is reduced by the cathodic protection; On the other hand, the Oxidation of the divalent iron hydroxide by the cathodic Corrosion protection even prevented. At cathodically protected plants lie consequently, the iron ions at least partially in divalent form. For detection and detection of AC corrosion can therefore be the detection of divalent iron ions utilized by an electrochemically controlled reaction become.

The working electrode with the defect is created by applying a voltage such anodically polarized that an oxidation reaction of existing divalent iron ions to trivalent iron ions occurs.

It is advantageous if a voltage drop in the electrolyte between the first and another electrode by detecting a Polarization potential difference determined and used as a correction variable.

In most cases of buried facilities or pipelines is included the application of the invention, the voltage drop in the electrolyte or the To consider soil. A determination of the true potential of Working electrode is not due to the falsification by this voltage drop always directly possible. However, a correction quantity can be determined by the current flow between the working electrode and the counter electrode is interrupted and the voltage difference between the working electrode and the reference electrode is determined immediately after the interruption.

Preferably, a measurement cycle is performed, wherein between the first and the at least one further electrode for a first period of time a voltage is created, and then the connection between the first and the at least separated another electrode for a second period of time and the Polarization potential difference is determined.

Typical time divisions for one measurement cycle are two to ten, e.g. five Seconds current flow between working electrode and counter electrode and 0.1 to 2 Seconds interruption. The measurement of the polarization potential difference, ie the thus detected voltage between the working electrode and the reference electrode, takes place for example, between 10 ns and 1 s after interrupting the flow of current.

It is advantageous if the measuring cycle is carried out periodically.

In development of the invention, at least one auxiliary electrode of the spaced electrode disposed and the auxiliary electrode via a electrical voltage source is electrically coupled to the first electrode in such a way that that an electric field effect of the first and the further electrode on a concentrated portion of the cathodic protected plant is concentrated.

Through this training, it can be ensured that the electricity is only off the defect to be examined emerges and the measurement of the AC corrosion considered only this flaw. By the concentration The electric field effect with an auxiliary electrode ensures that primary the defect in the pipeline is polarized and farther away defects are not contribute to the measurement result.

Preferably, as the auxiliary electrode, the further electrode is at a distance annular surrounding electrode used.

The auxiliary electrode is arranged annularly around the further electrode and limits the electric field effect between the first and the other Electrode applied voltage substantially to the inner annulus. This will remove distant imperfections of the plant, not by the measurement should be achieved, weaker polarized and participate in the coulometric Detection of the corrosion product at the defect not part.

Further advantageous embodiments of the invention will become apparent from the Dependent claims.

Figur 1 eine schematische Darstellung eines prinzipiellen Messaufbaus eines Ausführungsbeispiels der Erfindung;

Figur 2 ein Diagramm coulometrischer Messreihen zur Erfassung der Wechselstromkorrosion an Coupons unter Verwendung des Messaufbaus gemäß Figur 1;

Figur 3 ein Diagramm coulometrischer Langzeit-Messreihen zur Erfassung der Wechselstromkorrosion an einem erdvergrabenen Coupon;

Figur 4 eine schematische Darstellung eines abgewandelten Ausführungsbeispiels eines Messaufbaus für die Messung von Wechselstromkorrosion an einer Rohrleitung;

Figur 5 eine schematische Darstellung eines weiteren abgewandelten Ausführungsbeispiels der Erfindung.

In the following the invention will be explained in more detail with reference to the accompanying drawings. In the drawings shows:

Figure 1 is a schematic representation of a basic measurement structure of an embodiment of the invention;

FIG. 2 shows a diagram of coulometric measurement series for detecting AC corrosion on coupons using the measurement setup according to FIG. 1;

FIG. 3 shows a diagram of coulometric long-term measurement series for detecting AC corrosion on a ground-buried coupon;

Figure 4 is a schematic representation of a modified embodiment of a measurement setup for the measurement of AC corrosion on a pipe;

Figure 5 is a schematic representation of another modified embodiment of the invention.

FIG. 1 illustrates a simple exemplary embodiment of a measurement setup for detecting AC corrosion, using a three-electrode design is used. A working electrode 1, a reference electrode 2 and a Counter electrode 3 are connected to each other via an electrolyte 4 in electrolyte communication. Between the working electrode 1 and the reference electrode 2 is a Sensor arranged in the form of a voltage measuring device 6. A Voltage source 5 is on the one hand with the working electrode 1 and on the other hand with the Counter electrode 3 connected. A setpoint voltage or setpoint potential difference is by means of the voltage source 5 between the working electrode 1 and the Reference electrode 2 applied. It comes between the working electrode 1 and the counter electrode 3 to a current flow through the electrolyte 4 and to a electrochemical reaction at the working electrode 1 or the counter electrode 3. The counter electrode 3 may be formed as a galvanized earth spike, in the the Electrolyte forming soil 4 is driven. But it can also be one instead Pipe or a grounding system of a building as a counter electrode be used. Preferably, the surface of the counter electrode 3 significantly larger than that of the working electrode 1. The electrolyte 4 may be made Soil, seawater or the like exist.

FIG. 2 shows in a diagram the measurement results of coulometric measurements of corrosion products on test pieces (coupons). The measurements were made with a measuring setup shown in FIG. The measured coupons had an area of about 1 cm ² and were stored in an electrolyte consisting of artificial soil for seven days. The coupons were protected cathodically and subjected to different AC densities. The AC density on coupon (a) was 0 A / m ² and on coupon (b) 100 A / m ² . Before the measurement, the coupons were disconnected from the cathodic protection and the AC source. For the measurement, a constant anodic current of 100 μA was impressed between the coupon used as working electrode 1 and the counter electrode 3 with the aid of a galvanostat (voltage source 5). The potential difference was determined by voltage measurement between the working electrode 1 and the reference electrode 2 with a voltmeter 6. The control of the galvanostat and the detection of the polarization potential was performed by a computer. The measurement time for coupon (b) was 330 seconds. From the graph shown in the diagram, it can be seen that the potential initially increases with increasing charge flow and has a lower potential gradient in the region denoted by 7. In this area, the oxidation of divalent iron ions to trivalent iron ions takes place. As the charge flow increased, a steep increase in potential, indicated by 8, was observed. This potential increase ends with the onset of oxygen evolution at 9. With further charge, the potential remains nearly constant.

In the potential course of coupon (a) of the corresponding increase 8 takes place and increase indicated at 10 for significantly smaller charge quantities. The Oxygen evolution 11 occurs at the same potential as coupon (b). To In the measurement, the coupons were optically examined. On coupon (a) were found no corrosion attacks while coupon (b) showed severe corrosion. The charge flow dependent position of the beginning of the steep rise of the Potentials in this measurement arrangement is representative of the amount of oxidized divalent iron and thus the extent of AC corrosion at the Coupons. Furthermore, from the application potential of the oxygen evolution 9 or 11 information about the pH can be obtained.

FIG. 3 shows a diagram of the measurement of AC corrosion on an earth-buried coupon that was electrically connected and buried for 2.5 years with a cathodically protected pipe. The AC density at the coupon was in the range of 120 A / m ² . Before the measurement, the connection of the coupon to the pipeline was interrupted. For the measurement, an anodic current of 50 mA was applied between the coupon as the working electrode 1 and the counter electrode 3 of a galvanostat 5. Since the voltage drop in the electrolyte was significant and had to be considered, the current was interrupted at periodic intervals of 5 seconds for one second each. The potential difference between the working electrode 1 and the reference electrode 2 was determined immediately before and after breaking with a voltmeter 6. The curve (c) shows the potential values recorded before the interruption, while the curve (d) shows the potential values after the interruption of the current. The curve (d), formed by the potential values which are not influenced by the electrolyte resistance, shows good agreement with the potential profile shown in FIG. An interruption of the current allows the determination of the potential of the coupon without disturbing influences of the voltage drop in the electrolyte.

Under real conditions and for a duration of 2.5 years used coupons required the designated 12 potential increase about 300 times more charge than the rise 8 of Figure 2.

If the coupon is reconnected to the pipeline after the measurement, then The cathodic protection of the pipe again acts on the coupon, and the in Course of measurement oxidized iron ions are reduced again. A Repeated measurement at a later date is with good reproducibility possible. The method is for the repeated measurement of the corrosion state a coupon suitable.

Figure 4 shows a schematic representation of a modified Embodiment of a measuring structure for detecting the AC corrosion on a defect forming the first electrode 1 'on a pipeline 16. Die Defect 1 ', the reference electrode 2 and the counter electrode 3 are located in Contact with the electrolyte 4 and are in communication with each other in electrolyte. A voltage source 5 allows the setting of a voltage or a current between the defect and the defect 1 'and the counter electrode 3, and trained as a voltmeter transducer 6 takes the Voltage between the reference electrode 2 and the defect 1 'on.

Due to the underground extent of the pipe 16 is the field or Power distribution unknown. It is not excluded that further defects in the pipe 16 with the counter electrode 3 are in electrolyte communication and contribute to the flow of electricity.

To the electric field effect on a surrounding area of the to narrow down the investigative defect 1 'is an auxiliary electrode 14 via an in a potentiostat 15 arranged voltage source with the pipe 16 coupled. A second reference electrode 13 is connected to the potentiostat. The auxiliary electrode 14 annularly surrounds the counter electrode 3 and the Reference electrode 2. The auxiliary electrode 14 is further arranged such that also the defect 1 'is located in the inner annulus, in buried installations but mostly below the ring plane. Such an annular electrode is Also called Guardring. To the field effect on the area within the To limit Guardrings, the potential outside the ring with the Reference electrode 13 measured and a voltage between the guard ring 14th and the pipe 16 is controlled such that the of the reference electrode 13th measured potential is kept substantially constant. The between the Guard ring 14 and the pipe 16 applied voltage calls one resistance-dependent current flow. This corresponds to the partial flow of the counter electrode 3 flows into removed defects. The guard ring 14 may be off a single, planar electrode or a plurality of interconnected Be formed electrodes.

Figure 5 shows a schematic representation of another modified Embodiment of the invention. With two reference electrodes 18 and 19 is a Voltage drop in the electrolyte 4 detected by a current flow of the Counter electrode 3 is caused to distant defects of the pipe 16. One Controller 17 is coupled to the reference electrodes 18 and 19 and controls one between the formed as a guard ring auxiliary electrode 14 and the pipe 16 applied voltage such that the voltage between the reference electrodes 18 and 19 is set to 0V. As a result, the electric field effect on limits an area within the guard ring.

Within the scope of the inventive concept further modifications are possible. For example, a plurality of individual electrodes may be used as the auxiliary electrode be suitably in the direction of extension of the pipeline be arranged to limit the field effect. Furthermore, for detection the corrosion can be used any electrochemical reaction in which a corrosion product is involved. In the context of the invention may also be a Coulometric detection of a reduction of a corrosion product done. So can reduce the amount of corrosion products in the presence of a completely oxidized Surface also by coulometric measurement of the reduction of Fe (III) to Fe (II) be determined. The choice of reaction or the given electrochemical Controlled variables become cathodic depending on the material protected plant, as well as the surrounding electrolyte.

Claims (20)

A method of detecting AC corrosion on cathodically protected equipment, wherein a portion of the equipment to be tested contains a corrodible material which reacts to a corrosion product when AC corrosion occurs,
characterized in that from a part of the plant a first electrode is formed, which contains a quantity of the corrosion product,
that the first electrode is brought into electrolyte communication with at least one further electrode via an electrolyte,
in that an electrical voltage is applied between the first and the at least one further electrode in order to initiate an electrochemical reaction,
wherein the corrosion product of the first electrode participates in the electrochemical reaction and is converted to a reaction product,
wherein the course of the reaction is controlled by adjusting at least one electrical quantity at the electrodes,
wherein at least one electrical measured quantity is detected from a group of measured variables consisting of current intensity, potential difference and charge,
wherein the amount of reacted in the reaction corrosion product is determined from the at least one measured variable. A method according to claim 1, characterized in that in addition to the detection of the at least one electrical measured variable detects a measuring time and the amount of the corrosion product is determined taking into account the measuring time. A method according to claim 1 or 2, characterized in that a test piece (coupon) temporarily connected to the section to be examined is used as the first electrode. Method according to one of claims 1 to 3, characterized in that a portion of a cathodically protected pipe is used as the first electrode. Method according to one of claims 1 to 4, characterized in that the voltage between the first and the at least one further electrode is kept substantially constant. Method according to one of claims 1 to 4, characterized in that the current intensity between the first and the at least one further electrode is kept substantially constant. Method according to one of claims 1 to 6, characterized in that for controlling the electrochemical reaction, three electrodes are used in a three-electrode electrochemical structure with working, counter and reference electrode, wherein measured between the working electrode and the reference electrode, an electrical potential difference becomes. Method according to one of claims 1 to 7, characterized in that in at least a partial reaction of the electrochemical reaction, an oxidation of Fe ²⁺ to Fe ³⁺ or a reduction of Fe ³⁺ to Fe ^{2+ is} carried out. Method according to one of claims 1 to 8, characterized in that a voltage drop in the electrolyte between the first and another electrode is determined by detecting a polarization potential difference and used as a correction variable. A method according to claim 9, characterized in that a measuring cycle is carried out, wherein
a voltage is applied between the first and at least one further electrodes for a first period of time, and thereafter
the connection between the first and the at least one further electrode is separated for a second time duration and the polarization potential difference is determined. A method according to claim 10, characterized in that the measuring cycle is carried out periodically. Method according to one of claims 7 to 11, characterized in that at least one auxiliary electrode is arranged at a distance from the first electrode,
in that the auxiliary electrode is electrically coupled to the first electrode via an electrical voltage source in such a way that an electric field effect of the first and the further electrode is concentrated on a limited section of the cathodically protected plant. A method according to claim 12, characterized in that an electrode which surrounds the further electrode at a distance is used as auxiliary electrode. Arrangement for detecting AC corrosion on sections of cathodically protected installations, with
a first electrode (1), at least one further electrode (3) and a measuring transducer (6) for an electrical quantity which is electrically coupled to at least one of the two electrodes (1, 3),
wherein between the first electrode (1) and the at least one further electrode an electrolyte (4) is arranged, over which the electrodes (1, 3) are in electrolyte communication,
characterized in that a voltage source (5) is provided between the first electrode (1) and the at least one further electrode (3) in order to initiate an electrochemical reaction at least at the first electrode (1). Arrangement according to claim 14, characterized in that the first electrode (1) is a temporarily connected to the section of the system test piece (coupon). Arrangement according to claim 14, characterized in that the first electrode (1) is a section of a cathodically protected pipeline (16). Arrangement according to one of Claims 14 to 16, characterized in that the voltage source (5) used is an electrical device made up of the group consisting of a potentiostat, a galvanostat and a DC power supply. Arrangement according to one of claims 14 to 17, characterized in that a reference electrode (2) is provided, which is in electrolyte communication via the electrolyte (4) to the first electrode (1). Arrangement according to one of claims 14 to 18, characterized in that at least one auxiliary electrode (14) is provided which is electrically coupled to the first electrode (1) via an electrical voltage source (15) in order to produce an electric field effect of the first (1). and the further electrode (3) to concentrate on a limited portion of the cathodically protected plant. Arrangement according to one of claims 14 to 19, characterized in that the auxiliary electrode (14) is annular and is arranged at a distance around the further electrode (3).

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